One characteristics of Wideband Code Division Multiple Access/High Speed Packet Access (WCDMA/HSPA) is that downlink transmission to a specific user equipment is only performed by one radio base station, referred to as the serving radio base station. Evidently it is only the serving radio base station that benefits from knowledge of the physical layer information (Hybrid Automatic Repeat reQuest (HARQ) acknowledgement and Channel Quality Information/Pre-Coding Information (CQI/PCI)) transmitted on the High Speed Dedicated Physical Control Channel (HS-DPCCH) by the user equipment and thus only the serving radio base station decodes the HS-DPCCH.
Uplink data transmission from a user equipment can, in WCDMA/HSPA, on the other hand be received by multiple radio base stations. The set of radio base stations that decode the data transmissions from a particular user equipment constitute the active set for that user equipment. Some of the uplink related control channels are transmitted by all radio base stations in the active set, while some other control channels are transmitted by the serving radio base station only. One of the control channels transmitted by all radio base stations in the active set is the Fractional Dedicated Physical Channel (F-DPCH), which is used to control the transmit power of the user equipment. More specifically, transmit power control (TPC) commands sent on the F-DPCH adjust the Dedicated Physical Control Channel (DPCCH) transmit power of the user equipment.
The DPCCH transmit power is used as reference for all other physical channels transmitted by the user equipment. The power ratio between different physical channels is constant, which means that a change in the DPCCH transmit power will result in that the transmit powers of all other physical channels is also changed.
For a user equipment in soft handover (SHO) the transmit power on the uplink will be controlled by the radio base station associated with the highest Signal-to-Interference Ratio (SIR). FIGS. 2A and 2B schematically illustrate two examples of situations where a non-serving radio base station 20 would control the uplink power of the user equipment 300: 1) if the measured uplink interference level at the serving radio base station 10 is higher than the one measured by the non-serving radio base station 20 (FIG. 2A); and 2) if the pilot (DPCCH) power of the downlink is lower in the non-serving radio base station 20 than in the serving radio base station 10 (FIG. 2B). The latter can be a result of that the uplink link budget towards the non-serving radio base stations 20 is stronger than the link budget towards the serving radio base station 10.
In FIG. 2A, the interference level originating from another user equipment 30 can result in that the SIR target is only met by the non-serving radio base station 20. In FIG. 2B, the user equipment 300 will be power controlled by the non-serving radio base station 20 when the Common Pilot Channel (CPICH) power is lower for the non-serving radio base station 20 than for the serving radio base station 10 due to downlink asymmetry and the fact that the sending cell is based on downlink measurements. In both these cases the transmit power of the user equipment 300 is adjusted so that the quality at the non-serving radio base station 20 meets the desired target.
A consequence when the non-serving radio base station is controlling the uplink power is that the HS-DPCCH at the serving radio base station may become so weak that it cannot be correctly decoded or even detected. This can be a consequence of i) the received HS-DPCCH power at the serving radio base station is too weak and/or ii) the DPCCH quality at the serving radio base station is so weak that an adequate channel estimate cannot be derived at the serving radio base station.
The HS-DPCCH carries the HARQ acknowledgement and CQI/PCI related to the downlink transmission of the user equipment. This information is used by the serving radio base station to 1) decide how much information to transmit in a given Transmission Time Interval (TTI) and 2) decide whether a packet needs to be retransmitted. Inferior HS-DPCCH quality can thus result in that:                The HARQ acknowledgements indicating whether the user equipment was able to decode transmitted downlink transport blocks, are not detected or erroneously decoded. This will result in unnecessary layer 1 (L1) as well as Radio Link Control (RLC) retransmission.        The accuracy and availability of the CQI/PCI is reduced.        
Hence there are good reasons for ensuring a sufficient HS-DPCCH quality at the serving radio base station.
To combat these effects in existing solutions the HS-DPCCH typically has a higher power offset in SHO than in non-SHO situations.
On top of this the network has the possibility to order the user equipment to always repeat the HARQ Acknowledgements/Negative acknowledgements (ACKs/NACKs) on the HS-DPCCH. The drawback with this solution is however a reduction in the achievable downlink bit rate since a HARQ ACK/NACK repetition factor of x results in that the network can only schedule packet transmission to the user equipment once every x:th TTI.
There is therefore a need for an efficient power control solution that can be applied to ensure the HS-DPCCH quality when a user equipment has more than one radio base station in its active set.
WO 2009/072945 describes a method and an arrangement of obtaining efficient power control during soft handover in a communication network system when a user equipment is in communication with two or more radio base stations over a radio interface on downlink and uplink channels. TPC commands are received from the two or more radio base stations on the downlink channels. The received TPC commands are analyzed and a power offset on the uplink channels is adjusted based on the analyzed TPC commands. This technique does not ensure that the DPCCH quality needed for channel estimation at the serving radio base station is sufficient.